1. Field of the Invention
The invention relates to a conveyance seat. Especially, the invention relates to a conveyance seat that includes a pair of side frames arranged as seat back frames on both left and right sides, and a lower frame connecting lower positions of the side frames with each other.
2. Description of Related Art
A conveyance seat of an automobile and so on is made of a seat cushion serving as a seating section for a seated person, a seat back serving as a backrest, and a head rest supporting a head. The seat back has a seat back frame that forms a skeleton of the seat back. A pair of side frames arranged on both left and right sides, an upper frame that connects upper positions of the side frames with each other, and a plate-like lower frame that connects the lower positions of the side frames with each other are arranged into a frame shape, thus structuring the seat back frame. The seat back is able to move in a front-back direction through reclining mechanisms provided in a lower position of the seat back, and is connected with the seat cushion. When positioning mechanisms of the reclining mechanisms are arranged on both left and right sides, an interlocking rotating shaft is arranged in parallel with the above-mentioned lower frame in order to interlock the positioning mechanisms on both sides. The lower frame and the interlocking rotating shaft are positioned to each other as follows. The lower frame is formed to have a U-shaped section, and is arranged so that an opening of the U shape comes to the top. Then, the interlocking rotating shaft is positioned inside the U shape. Thus, even if support load of a seat pad arranged in front of the interlocking rotating shaft acts on the interlocking rotating shaft, the support load of the seat pad does not directly acts on the interlocking rotating shaft, and there is no hindrance in an operation of the reclining mechanisms. In other words, when there is no front side area of the U-shaped lower frame, support load of the seat pad is received by the interlocking rotating shaft as well as a rear side area of the lower frame. In such a case, for example, at a time of rear collision and so on of a conveyance, the seat pad could apply force to the interlocking rotating shaft in a rotating direction due to load received by the seat pad, and the reclining mechanisms could be released carelessly. Also, in a normal state, support load of a seated person applied to the seat pad always acts on the interlocking rotating shaft while the seated person is seated. Therefore, when the reclining mechanisms operate, the support load acts as resistance while the interlocking rotating shaft rotates. In order to solve this problem, the lower frame is formed to have a U-shape section so that support load of the seat pad does not act on the interlocking rotating shaft.